This citizen journalism image provided by the Local Committee of Arbeen which has been authenticated based on its contents and other AP reporting, shows a wounded Syrian girl waiting for treatment after an alleged poisonous gas attack fired by regime forces, according to activists in Arbeen town, Damascus, Syria, Wednesday, Aug. 21, 2013. Syrian regime forces fired intense artillery and rocket barrages Wednesday on the eastern suburbs of the capital Damascus, in what two pro-opposition groups cl

BEIRUT - Syrian regime forces fired intense artillery and rocket barrages Wednesday on the eastern suburbs of the capital, Damascus, in what two pro-opposition groups claimed was a "poisonous gas" attack that killed dozens of people.

The claims came as a 20-member U.N. chemical weapons team was in Syria to investigate three sites where chemical weapons attacks had allegedly occurred in the past.

The Syrian government promptly denied the reports of Wednesday's chemical weapons' attack as baseless.

"They are an attempt by to divert the U.N. commission on chemical weapons from carrying out its mission," the state-run SANA news agency said, quoting an unnamed government official, as is its standard practice.

The Britain-based Syrian Observatory for Human Rights said the shelling was intense and hit the capital's eastern suburbs of Zamalka, Arbeen and Ein Tarma.

It quoted activists as saying that regime forces fired "rockets with poisonous gas heads" in the attack that killed "tens of people."

Rami Abdul-Rahman, the Observatory chief, said the activists in the area said "poisonous gas" was fired in rockets as well as from the air. He added that regime forces were on a wide offensive on the eastern and western rebel-held suburbs of Damascus.

Another group, the Local Coordination Committees, said hundreds of people were killed or injured in the shelling. Such different figures from activists groups are common in the immediate aftermaths of attacks in Syria, where the government restricts foreign and domestic reporting.

Abdul-Rahman said more than 40 people have been confirmed dead and that the death toll could reach as many as 200 in the suburbs of Damascus.

The Syrian government has long denied claims by the opposition on chemical weapons use, saying rebels fighting to overthrow President Bashar Assad's government have used such weapons.

Following Wednesday's reports, the Observatory called upon the U.N. team in Syria and all international organizations "to visit the stricken areas and to guarantee that medical and relief supplies reach the people as soon as possible." It also called for an investigation into the attack.

Mohammed Saeed, an activist in the area, told The Associated Press via Skype that hundreds of dead and injured people were rushed to six makeshift hospitals in the eastern suburbs of Damascus.

"This is a massacre by chemical weapons," said Saeed. "The visit by the U.N. team is a joke ... Bashar is using the weapons and telling the world that he does not care."

The U.N team, led by Swedish expert Ake Sellstrom, is meant to probe three sites: the village of Khan al-Assal just west of the embattled northern city of Aleppo and two other locations, which are being kept secret for security reasons.

Wednesday's claim of the chemical attack, if confirmed, would be the most serious since the March 19 incident in Khan al-Assal when at least 30 people were killed. Assad's regime and the rebels have blamed each other for that attack.

Unrest in Syria began in March 2011 and later exploded into a civil war. More than 100,000 people have been killed in the conflict.

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Syria War In July (WARNING: GRAPHIC IMAGES)

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A Syrian rebel tries on a gas mask seized from a Syrian army factory in the northwestern province of Idlib on July 18, 2013. (DANIEL LEAL-OLIVAS/AFP/Getty Images)

Syrian volunteers try to evacuate 8-year-old Mahmud Fayad still trapped under the rubble of a destroyed house after a barrel bomb was dropped from an air force helicopter in Saraqeb in northwestern Syria on July 20, 2013. (DANIEL LEAL-OLIVAS/AFP/Getty Images)

In this photo released by the Syrian official news agency SANA, Syrian citizens gather near damaged cars that were burned after a car bomb exploded in the suburb of Jaramana, Damascus, Syria, Thursday, July 25, 2013. (AP Photo/SANA)

An Indian Shiite Muslim woman breaks down as she holds a placard during a protest rally organized to condemn an attack on a Shiite shrine in Syria, in Bangalore, India, Friday, July 26, 2013. (AP Photo/Aijaz Rahi)

A Syrian man stands amidst the rubble of the Othman mosque, in Syria's eastern town of Deir Ezzor, on July 24, 2013. (KARAM JAMAL/AFP/Getty Images)

An injured Syrian man receives medical treatment at a hospital in the town of Ariha, in the northwestern province of Idlib, on July 21, 2013. (DANIEL LEAL-OLIVAS/AFP/Getty Images)

This Tuesday, July 9, 2013 citizen journalism image provided by Aleppo Media Center AMC, which has been authenticated based on its contents and other AP reporting, shows buildings are seen damaged by Syrian government airstrikes and shelling, in the Salah al-Din neighborhood of Aleppo, Syria. (AP Photo/Aleppo Media Center AMC)

Syrian volunteers cry of joy after rescuing 8-year-old Mahmud Fayad who was trapped under the rubble of a destroyed house after a barrel bomb was dropped from an air force helicopter in Saraqeb in northwestern Syria on July 20, 2013. (DANIEL LEAL-OLIVAS/AFP/Getty Images)

In this photo released by the Syrian official news agency SANA, Syrian citizens gather near damaged cars that were burned after a car bomb exploded in the suburb of Jaramana, Damascus, Syria, Thursday, July 25, 2013. (AP Photo/SANA)

Syrians inspect the site where a barrel bomb dropped by an air force helicopter exploded in Saraqeb in northwestern Syria on July 20, 2013. (DANIEL LEAL-OLIVAS/AFP/Getty Images)

Syrian men rush a heavily wounded man to hospital in Saraqeb in northwestern Syria after a barrel bomb dropped by an air force helicopter exploded less than 10 metres away from his car on July 20, 2013. (DANIEL LEAL-OLIVAS/AFP/Getty Images)

Indian Shiite Muslim women raise slogans as they participate in a rally organized to condemn an attack on a Shiite shrine in Syria, in Bangalore, India, Friday, July 26, 2013. (AP Photo/Aijaz Rahi)

In this photo released by the Syrian official news agency SANA, Syrian citizens look at a damaged building after a car bomb exploded in the suburb of Jaramana, Damascus, Syria, Thursday, July 25, 2013. (AP Photo/SANA)

Syrians search for survivors under the rubble of a destroyed house after a barrel bomb was dropped from an air force helicopter in Saraqeb in northwestern Syria on July 20, 2013. (DANIEL LEAL-OLIVAS/AFP/Getty Images)

Doctors treat a young Syrian girl hit by shrapnel following an air raid in the northern Syrian town of Saraqeb, in the Idlib province, on July 19, 2013. (DANIEL LEAL-OLIVAS/AFP/Getty Images)

A Syrian girl walks next to the debris of her house, after it was targeted by a missile, in the northwestern Syrian province of Idlib on July 18, 2013. (DANIEL LEAL-OLIVAS/AFP/Getty Images)

In this Wednesday, July 3, 2013 citizen journalism image provided by Lens Young Homsi, which has been authenticated based on its contents and other AP reporting, shows Syrians inspecting the rubble of damaged buildings due to government airstrike and shelling, in the al-Hamidiyyeh neighborhood of Homs province, Syria. (AP Photo/Lens Young Homsi)

A picture taken on July 18, 2013 shows smoke rising from a building after opposition forces attacked a government forces held checkpoint in Syria's northeastern city of Deir Ezzor on July 18, 2013. (ABO SHUJA/AFP/Getty Images)

This aerial view shows the Zaatari refugee camp on Thursday, July 18, 2013 near the Jordanian city of Mafraq, some 8 kilometers (5 miles) from the Jordan-Syria border. (AP Photo/Mandel Ngan, Pool) AP10ThingsToSee

A rebel fighter points his gun towards pro-government forces' positions during clashes in the Salaheddine district of the northern Syrian city of Aleppo, on July 9, 2013. (JM LOPEZ/AFP/Getty Images)

Syrian children wait in line to collect a free Iftar meal in the northern city of Raqqa during the Muslim holy month of Ramadan on July 14, 2013. (MEZAR MATAR/AFP/Getty Images)

A rebel fighter aims his weapon to Syrian government forces' positions inside a destroyed school in the Izaa district of Aleppo, on July 14, 2013. (JM LOPEZ/AFP/Getty Images)

A rebel fighter from the Suqur Al-Sham brigades, takes a nap after his night shift in a trench along the highway that connects Idlib city with Latakia, 100m far from the Syrian regime's forces's positions, on July 14, 2013. (DANIEL LEAL OLIVAS/AFP/Getty Images)

In this citizen journalism image provided by Aleppo Media Center AMC, which has been authenticated based on its contents and other AP reporting, buildings are seen damaged by Syrian government airstrikes and shelling in Aleppo, Syria, Monday, July 8, 2013. (AP Photo/Aleppo Media Center AMC)

Family members pray over the bodies of two Syrian children, Safia Baour, 14, and her three-year-old brother Khaled, who were killed after a shell landed on their family home as they gathered to break their fast with the iftar meal on July 13, 2013 in Maaret Al-Numan in southern Idlib province. (DANIEL LEAL-OLIVAS/AFP/Getty Images)

Syrian rebels head to the town of Bsankol in the northwestern province of Idlib to join comrades fighting regime forces for the control of the highway that connects Idlib with Latakia on July 11, 2013. (DANIEL LEAL-OLIVAS/AFP/Getty Images)

An opposition fighter stands over seven year old Ahmad Jabir, who was injured alongside some his family members by a shell, as he lies on a X-Ray machine after he brought the boy to a hospital in the town of Al-Bara, in Syria's northwestern province Idlib on July 10, 2013. (DANIEL LEAL-OLIVAS/AFP/Getty Images)

Syrian rebels gather outside a building which they blew up to target a regime sniper taking shelter inside but who managed to escape in the Salaheddine district of the northern city of Aleppo on July 10, 2013. (SALAH AL-ASHKAR/AFP/Getty Images)

Rebel fighters look up listening as they take positions during clashes with pro-government forces in the Salaheddine district of the northern Syrian city of Aleppo, on July 9, 2013. (JM LOPEZ/AFP/Getty Images)

This Tuesday, July 9, 2013 citizen journalism image provided by Aleppo Media Center AMC, which has been authenticated based on its contents and other AP reporting, shows Syrian rebels running during heavy clashes with Syrian soldiers loyal to Syrian President Bashar Assad, in the Salah al-Din neighborhood of Aleppo, Syria. (AP Photo/Aleppo Media Center AMC)

Rebel fighters break a wall before holding a position during clashes with pro-government forces in the Salaheddine district of the northern Syrian city of Aleppo, on July 9, 2013. (JM LOPEZ/AFP/Getty Images)

A rebel fighter speaks on a walkie-talkie as he holds a position in a flat during clashes with pro-government forces in the Salaheddine district of the northern Syrian city of Aleppo, on July 9, 2013. (JM LOPEZ/AFP/Getty Images)

Syrian rebel fighters gather around a former Syrian army tank as rebels prepare to attack positions held by the Syrian army areas in the Salaheddine neighborhood of Aleppo, on July 8, 2013. (ABO MHIO/AFP/Getty Images)

Opposition fighters carry one of their comrades, who died on the front-line, as they were leading an attack on positions held by the government forces in the Salaheddine neighbourhood of Aleppo on July 8, 2013. (ABO AL-NUR SADK/AFP/Getty Images)

An opposition fighter throws a Molotov cocktail during clashes with Syrian government forces in the Salah al-Din area of the northern city of Aleppo on July 8, 2013. (ZAID REV/AFP/Getty Images)

In this Tuesday, July 2, 2013 citizen journalism image provided by Aleppo Media Center AMC, which has been authenticated based on its contents and other AP reporting, homes are seen destroyed by Syrian government airstrikes and shelling in the neighborhood of Karam Tarab near Aleppo International Airport in Aleppo, Syria. (AP Photo/Aleppo Media Center AMC)